A cloud chamber is an instrument to display tracks of nuclear radiation - alpha, beta and gamma rays. A space is filled with vapour. A piston expands, the gas in the chamber is cooled and the temperature falls below the condensation point of the vapour. Charged particles or energetic photons passing through the chamber collide with gas molecules, creating ions that provide condensation points. Condensation forms along the path of the nuclear radiation.
The type of radiation can be identified in two ways: Alpha particles produce thick short tracks and curl slowly one way in a magnetic field ant right angles to the chamber.
Beta particles produce thinner longer tracks and curl more quickly the opposite sense in a magnetic field at right angles to the chamber. Gamma rays produce wispy tracks the length of the cloud chamber and are undeflected by a magnetic field.